1. Field of the Invention
This invention relates to a process for the electrolytic coloring of anodized surfaces of aluminum or aluminum alloys using alternating current or direct current superimposed on alternating current, the electrolytic coloring being carried out with an electrolyte containing cationic organic dyes.
2. Statement of Related Art
To increase resistance to corrosion and to obtain decorative effects, the surface of aluminum and its alloys may be substantially modified by mechanical techniques or may be provided with metallic or non-metallic coatings. Reinforcement of the natural protective oxide film by chemical or electrical techniques has acquired considerable significance.
In the prior art, processes for coloring surfaces of aluminum or aluminum alloys comprise adsorptive coloring, color anodizing and electrolytic coloring, see Wernick, Pinner, Zurbruegg, Weiner "Die Oberflaechenbehandlung von Aluminium (The Surface Treatment of Aluminum)", Leuze Verlag, (pub.) Saulgau, Wuertt (1977), pages 354 to 374 and 309 to 312.
In adsorptive coloring, for example, an organic dye is introduced into the pore openings of the oxide layer, remaining adsorbed in the surface region of the surface. Adsorptive coloring enables the entire color spectrum to be obtained with a high degree of uniformity and reproducibility. The various dyes useable in this process are commercially obtainable.
In addition, color anodizing (integral method) has been in use for years. In a substrate colored by the integral method, the finely divided inorganic dye particles are not situated in the pores of the oxide layer, but remain behind as an alloying constituent in the aluminum oxide layer. In the integral process, special aluminum alloys are both electrolytically oxidized and also colored in a single process step, generally using d.c. voltages of up to 150 V. The electrolyte used consists of a suitable organic acid, for example maleic, oxalic, sulfosalicylic, or sulfophthalic. However, the integral process is being used increasingly less in practice for reasons of cost (high current consumption, expensive cooling systems).
By contrast, in electrolytic coloring using metal salt solutions, a colorless transparent oxide layer is produced in a first process step by anodic oxidation using direct current in aqueous sulfuric acid and/or other electrolyte solutions. In a second process step, it is colored (in contrast to adsorptive coloring) by deposition of metal particles on the bottom of the pores in the oxide layer from metal salt solutions using alternating current. The colors range from light bronze through dark bronze to black. Completely light-stable color finishes are obtained because the coloring metal particles are incorporated on the bottom of the pores (W. Sautter, Metalloberflaeche, 32, 1978, pages 450 to 454).
By virtue of their advantages, such as relatively high light stability and weather resistance, electrolytic coloring processes are largely used for coloring aluminum which is to be used in the architectural field. Electrolytic coloring processes are dominated by electrolytic metal salt coloring by virtue of its relatively low costs and, thus, greater economy compared with integral coloring, Sn(II)-, Co-, Ni- and Cu-containing electrolyte solutions preferably being used in electrolytic metal salt coloring.
U.S. Pat. 4,401,525 (and corresponding published German application No. 28 50 136) describe a process for the electrolytic metal salt coloring of aluminum in which a defined oxide layer is first produced by direct current in acidic solution and subsequently colored using alternating current and an acidic electrolyte containing tin(II) salts, the electrolyte also containing stabilizers for the tin(II) salts. However, coloring electrolytes containing metal salts such as these are unsuitable for producing brightness and lightness of any degree on the surfaces of aluminum and aluminum alloys.
Published German patent application No. 32 48 472 describes a process for coloring anodically produced oxide coatings on aluminum and aluminum alloys which uses a coloring electrolyte with which it is possible to obtain colors of different brightness and lightness ranging from gray through bronze to violet-blue, more especially for use in profiles for windows, doors, facade panels and the like, on anodized aluminum surfaces. To enable color finishes such as these to be economically and reproducibly obtained in the same color at any time, even where different shades are involved, the coloring electrolyte contains an organic dye component in addition to a metal salt. An azo dye containing metal complexes is proposed as the organic dye component. Thus, published German application No. 32 48 472 describes a process for coloring anodically produced oxide coatings in an electrolyte containing metal salts with simultaneous adsorptive coloring using an azo dye containing metal complexes.
None of the coloring processes described above are entirely satisfactory in terms of practical application. Electrolytic coloring processes (including both the integral process and also metal salt coloring) do not produce bright colors, but only gray or bronze to black. Although a wide range of bright colors can be obtained by adsorptive coloring, the dyes used are only adsorbed in the upper region of the pores. Accordingly, the color finishes are not abrasion-resistant. Under mechanical stressing, the surface is attacked, i.e. the dyes are worn away so that the color is lost. Since stressing of the type in question is locally irregular, the resulting scratches, marks, discoloration and the like are particularly noticeable. Accordingly, the usefulness of aluminum parts colored in this way is seriously affected. Surface coloring of the type in question is also unsuitable for aluminum facade panels because their subsequent cleaning with preparations normally containing abrasives also results in fading.